Method for forming a protective facing on a cathode ray tube



y 1967 v. E. HAMILTON 3,329,545

METHOD FOR FORMING A PROTECTIVE FACING ON A CATHODE RAY TUBE Filed March 6, 1964 34 I 24 o E 50 /3 s 52 y g 42 4% I l T I I I I1g.L 45

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Filed Mar. 6, 1964, Ser. No. 350,011 3 Claims. (Cl. 156213) This invention relates to the art of shaping a sheet of plastic mate-rial to correspond substantially exactly with the shape of the front wall of a cathode ray tube and permanently securing the sheet in place to produce, with the glass of the tube, a safety glass combination to prevent shattering in the event of an implosion or other breakage of the tube. More particularly it relates to :a rnehod and apparatus for forming the plastic sheet in situ on the face of the tube, and cementing it thereto either separately or in the same operation.

Many present day cathode ray tubes, particularly those for use in television receivers, are provided with plastic sheets cemented directly to their front faces. The combination constitutes a two-piece safety glass to prevent shattering in case of breakage. It is desirable that the contour of the sheet match that of the glass Wall as exactly as possible in order to minimize cement gaps and distortion, When the sheets are molded independently of the tubes, the variations in curvature cause considerable difficulty in matching the surfaces well enough to eliminate the difficulties mentioned above. One solution which has been used for some time with reasonable success has been to form each sheet on the tube with which it is to be used, the front wall of the tube serving as a forming die. This has been accomplished by positioning :a CR tube in a closed, vacuum-tight framework having a clamping ring at the top, with the front wall of the tube facing upwardly. A sheet of plastic material is gripped in the clamping ring and the chamber evacuated. Then a hydraulic jack, mounted on the base of the framework, pushes the tube upwardly to contact and stretch the sheet which is also being drawn down by the vacuum. The tube is moved upwardly until the sheet contacts all of the front wall of the tube and is curved down over a small portion of the side wall. The assembly is heated during the process to soften the pastic.

There has remained a need for a more convenient system which will automatically insure accurate matching and bonding of the sheet to the tube wall without the hum-an errors inherent in trying to coordinate the various factors of (1) degree of softness of the plastic, (2) draw-down action by the vacuum, (3) wrap-around action by movement of the CR tube with the hydraulic jack. The present invention accomplishes these ends fully automatically with very little skill required of the operator. In one preferred form, the apparatus of the invention comprises a base and a surrounding bellows type or telescoping side wall forming a container having a closed end and an open end. A fixture is provided to support a CR tube with its front wall facing outwardly adjacent the open end of the container. Means in the form of clamping frame members are connected sealingly to the open end of the side wall and grip a sheet of plastic in sealing engagement to complete an enclosure surrounding the tube.

The axial length of the enclosure or container is variable, and in one form this is accomplished by making the length of the side wall variable. It may have a rigid portion and a collapsible portion, preferably of bellows formation, -or it may have telescoping sections. Also the wall may be of fixed length and the clamping frame members may move axially of the wall in sealing engagement.

3,329,545 Patented July 4, 1967 However, the bellows formation is simpler and more satisfactory in general and is now the preferred form. When the enclosure is completed, a vacuum pump connected to the interior of the enclosure is actuated and a vacuum is produced within the container. The bellows wall has a very low spring rate so that it will collapse axially under a very small load. Hence, the vacuum produced readily collapses the wall and draws the sheet toward the tube. Heat is applied to soften the sheet and differential air pressure forces it into intimate contact with the front wall of the tube so that their shapes cor respond exactly. When the sheet is cooled it will retain its new shape. It should be noted that the action is fully automatic with the drawing of the Vacuum and the heating of the plastic, since the wrap-around action will not begin until the plastic softens, and further the wraparound does begin and completes before the plastic gets too soft. It is readily seen that in previous methods the operator could force the CR tube into the plastic before it had properly softened, or could wait until the plastic was too soft to achieve a tight stretching action.

This operation can be performed with each sheet and each tube, and the cementing can be done subsequently. However, in order to simplify the process, the cementing is ordinarily done in the same operation. The cement may be applied to either of the confronting faces and may be a dry, heat softening type or a catalytic type. Alterna tivel-y, it may be a very thin sheet of a suitable plastic such as polyvinyl butyral which softens and becomes adhesive at elevated temperatures. Such sheet may be inserted between the members and will conform to the tube well during the forming and cementing operation. It will be seen that a squeezing out action is automatically generated when the vacuum is turned on with the plastic cold. The center contacts first, then as the plastic is heated wrap-around generates from the center outward and air bubbles are squeezed out from center to edge.

A radiant heater is located exteriorly of the assembly and applies heat only to the sheet to soften it and facilitates its conformation, and very little heat reaches the tube, only the front wall being in contact with any heated material. Even if it is elected to pass the assemblies through an oven, only a very slight amount of heat reaches the parts of the tube other than the front Wall because it is completely surrounded by the container and the plastic sheet, and the vacuum within the enclosure serves as a very effective insulator. Moreover, the heating time is rather short.

The plastic sheet is normally planar when secured to the container to reduce the total operations required. However, if desired, the sheets may first be domed to approximate the final shape and then formed exactly in the drawing operation. The sheets may be used simply for their safety glass feature but they may also incorporate the space lattice type filter disclosed in my copending application, Ser. No. 230,644, filed Oct. 15, 1962.

Various other advantages and features of novelty will become apparent as the description proceeds in coniunction with the accompanying drawing in which:

FIGURE 1 is a front elevational view, partly in section, of an apparatus embodying the invention just prior to the drawing operation;

FIGURE 2 is a view similar to FIGURE 1, showing the relation of the parts at the end of the drawing operation; and

FIGURE 3 is a top plan view of the apparatus of FIGURE 1.

The presently preferred embodiment of the invention as illustrated in FIGURE 1 includes a base or rear wall 10 to which is attached a surrounding side Wall 12 to constitute a container having an open upper or outer free end 14. The Wall may be entirely of bellows like formation but, as shown, the lower or rear portion 16 is rigid and the upper or forward portion 18 is in the form of a bellows having a low spring rate so that it may be collapsed readily under a relatively light load. Portion 16 is in sealing engagement with base and may be removable or fixedly attached.

A retainer 20 is sealingly engaged and preferably fixed to the upper end 14 of the wall to move axially with it in expansion and collapse. Ears 22, integral with the retainer, have bores slidably fitting on guide rods 24 extending outwardly from the base to maintain the retainer parallel to the base at all times. A clamping frame 26 is carried by the retainer and secured to it by a plurality of bolts 28. Sheet 30 of heat softenable plastic material extends across the opening in frame or retainer 20 and its margin 32 is firmly gripped between the retainer and flange 34 of clamping frame 26. When bolts 28 are properly drawn up, this becomes a sealed joint. A layer of cement 31 is applied to the inner surface of the sheet. Alternatively, it may be applied to the front wall of the tube. The cement may be heat softening or catalytic. In some cases it is desirable to insert between the members a separate thin sheet of suitable plastic material, such as polyvinyl butyral which becomes adhesive at elevated temperatures.

A radiant heater 36 of any suitable type, preferably in pancake form, is positioned to apply heat to the outer surface of sheet 30 to soften it sufficiently so that it will readily conform to the shape of the front wall 38 of CR tube 40. The latter is suitably mounted in a fixture 42 on base 10 so that it Will remain in the position shown throughout the operation. Conductors 44 lead to a source of electric power, not shown. A vacuum pump or tank 46, of suitable capacity, is connected by conduit 48 to the interior of the enclosure formed by base 10, wall 12, retainer 20, and sheet 30 and, when activated, it produces a vacuum within the enclosure which draws sheet 30 rearwardly toward base 10, collapsing wall portion 18 in the process.

As the heat softened sheet moves rearwardly, its center portion first contacts the center portion of tube wall 38 and, as the rearward movement continues, it is draped or drawn into intimate contact with wall 38 until it is drawn into the shape shown in FIGURE 2. The differential air pressure which is applied to its entire exposed surface provides uniform pressure at all points and helps to eliminate bubbles between the sheet and the glass, both by the squeezing action and the fact that the vacuum pumps away the gases which constitute the bubbles. In order to control the extent of the drawing action, stops in the form of collars 50 are provided on the guide rods and are held in position by set screws 52. The position of the stops is adjusted to provide the proper amount of movement with different sizes and types of tubes.

Whether the radiant heater shown is used or the assemblies are passed through an oven, the vacuum is maintained for a period of time while the sheet returns to room temperature, when it in firmly set in its new shape. By this time the cement will also have hardened or cured, and the tube and sheet may now be removed from the assembly. The excess portion of the sheet is now severed peripherally along a path indicated by lines 54, a strip of adhesive tape is applied around the joint to keep out moisture through the service life, and the safety glass combination is now completed and ready for use. In the case where my space lattice type filter has been incorporated in sheet 30 the tube now has both a safety glass combination and an integral ambient light trapping filter.

While the apparatus has been shown as generally cylindrical or circular in form to process the tube shown, it may have a shape generally corresponding to the planform of the front wall of the tube to be processed.

It will be apparent to those skilled in the art that various changes and modifications may be made in the method and apparatus disclosed without departing from the spirit of the invention, and it is intended that all such changes and modifications shall be embraced within the scope of the following claims.

I claim:

1. A method of conforming a sheet of plastic material to the front wall of a cathode ray tube, consisting of: disposing said sheet in closely spaced relation to said front wall and transverse to the axis of said tube; creating a pressure difference on the opposite faces of said sheet to cause it to move into contact with the central portion of said front wall; applying heat to said sheet to soften it; and continuing said pressure difference to cause said sheet to move rearwardly relative to said tube and gradua'lly increase its area of contact with the front wall in all radial directions from the central portion and drape itself over the adjacent side wall portions of said tube as the material of said sheet gradually softens.

2. A method as claimed in claim 1; including a preliminary step of doming the sheet to the approximate shape of said front wall to reduce the forming required in the subsequent steps.

3. A method of conforming a sheet of plastic material to the front wall of a cathode ray tube, consisting of: arranging said tube in a fixed position; disposing said sheet in closely spaced relation to said front wall and transverse to the axis of said tube; applying a force to the margin of said sheet in a direction to move it toward the rear of said tube until the sheet comes into contact with the central portion of said front wall; applying heat to said sheet to cause it to gradually soften; and continuing said force to cause the margin of the sheet to continue to move rearwardly and gradually draw the sheet into contact with the entire area of said front wall and the adjacent side wall portions of said tube as the material of said sheet gradually softens.

References Cited UNITED STATES PATENTS 3,060,507 10/1962 Knowles 1819 3,072,964 l/1963 Tilden 26492 3,264,157 8/1966 Lattimer 156-272 EARL M. BERGERT, Primary Examiner.

W. E. HOAG, Assistant Examiner. 

3. A METHOD OF CONFORMING A SHEET OF PLASTIC MATERIAL TO THE FRONT WALL OF A CATHODE RAY TUBE, CONSISTING OF: ARRANGING SAID TUBE IN A FIXED POSITION; DISPOSING SAID SHEET IN CLOSELY SPACED RELATION TO SAID FRONT WALL AND TRANSVERSE TO THE AXIS OF SAID TUBE; APPLYING A FORCE TO THE MARGIN OF SAID SHEET IN A DIRECTION TO MOVE IT TOWARD THE REAR OF SAID TUBE UNTIL THE SHEET COMES INTO CONTACT WITH THE CENTRAL PORTION OF SAID FRONT WALL; APPLYING HEAT TO SAID SHEET TO CAUSE IT TO GRADUALLY SOFTEN; AND CONTINUING SAID FORCE TO CAUSE THE MARGIN OF THE SHEET TO CONTINUE TO MOVE REARWARDLY AND GRADUALLY DRAW THE SHEET INTO CONTACT WITH THE ENTIRE AREA OF SAID FRONT WALL AND THE ADJACENT SAID WALL PORTIONS OF SAID TUBE AS THE MATERIAL OF SAID SHEET GRADUALLY SOFTENS. 